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Net-Zero Trifecta: Solar, Heat Pumps and Smart Controls

Posted by Matt Power, Editor-In-Chief

Feb 14, 2017 12:21:29 PM

Getting to net zero is within easy reach with new technologies. The only real hurdles are politics and ignorance.

A FEW MONTHS AGO, I spoke to builder Gene Myers of Thrive Home Builders in Denver about whether mini-split heat pumps could be powered by solar PV panels, essentially converting electricity into affordable, clean heating and cooling for homes.

“We’re doing that on all of our new homes,” Myers told me. “We have been for a couple years now.”

That took me by surprise.

At the time, I hadn’t looked closely at how such a marriage of technologies would work. Since then, I’ve been putting the pieces together, planning a system for my own building with a solar array and mini splits, all managed through a smartphone app.
What I’ve learned is that the path to net zero with solar heating/cooling is possible, but the devil really is in the details. Some of the devils, however, are elected officials.

net metering july 2016

Darker areas show where net metering allows owners to trade energy credits at retail value.

Indirect, Not Direct Power

It’s important to understand that solar PV panels do not directly power mini splits—at least, not yet. Even the best new DC-to-AC inverters, made by companies such as SMA, can’t overcome the inconsistency of how weather patterns affect PV power generation. According to Zach Nugent of ReVision Energy in Portland, Maine, weather and seasonal issues (along with the Earth’s rotation) mean that power output from PV is not consistent enough to take over the high demand of a mini-split system’s outdoor condensing unit. This limitation applies even to a super-efficient model with a seasonal energy efficiency ratio (SEER) of 15 or more and an energy efficiency ratio (EER) of 12.5 or more, with variable output.

For example, the outdoor component of my multi-zone (three mini split) Mitsubishi system draws about 3,800 watts at peak demand. My 6-kW solar array will sometimes deliver that wattage and more—but not consistently.

This need for a buffering technology narrows the options for residential solar, and also explains why so many companies are getting into the energy storage arena. PV’s path to heating and cooling—the biggest energy users in the home—is limited to two indirect routes: net metering or batteries.

heat pump chart 300.jpg

Net Metering: Under Siege

Grid-tied solar that feeds back into existing infrastructure is by far the least costly (and historically, fastest) path to mass market adoption of solar. It’s the simplest way to get to net zero using mini splits. While the homeowner isn’t technically feeding power directly to the HVAC system, the home is producing as much power as the units consume. Result equals net zero.

With net metering, power from the PV panels is fed back into the local grid, and typically the owner gets a credit. As the map shows, about one-third of states currently trade these credits at full retail value. Another third offer less than retail value, and the remainder offer little or no incentive for solar. So, where you live has a big impact on how fast your system pays for itself in energy savings, and how soon in the discussion you want to start looking at batteries.

Another major impact on net metering is the disposition of local politicians. Are they beholden to coal, oil or other fossil fuel interests? This is all too common. In other words, despite best efforts to get off fossil fuels, you remain at the mercy of fossilized thinking.

A perfect example is playing out here in Maine. Our governor, Paul LePage, a notorious foot-in-mouth bully who has been described by the Washington Post as “completely unhinged,” has joined the politically charged anti-solar movement launched by public utilities in several states. LePage has already vetoed one solar incentive bill, and is pushing for a rollback of net metering incentives. And he’s just one example. It’s a national confrontation between old and new technologies, with politicians acting as willing pawns.

Without getting too far into the weeds, political resistance to solar comes not only from fossil fuel industries, but also from utilities. They assert that non-solar customers end up paying for the grid infrastructure and PV owners don’t. That’s true at face value. But as solar advocates point out, infusing localized power into the grid has huge benefits for customers and utilities. It not only improves performance in high-demand periods, but can forestall construction of new plants. The pros of net metering are well documented at Media Matters for America, if you want to read up on the topic.

Let’s assume the worst: that metering gets taken off the table by political wrangling. Residential solar owners looking to power mini splits will be put in a frustrating situation. They’ll be producing plenty of power on their rooftops, with no way to transfer it to their equipment.

Batteries: (Almost) Ready for Prime Time

The solution, of course is batteries. The storage industry is finally catching up to the fast-growing solar PV universe. But this is a bleeding-edge industry. Any system you adopt will involve a substantial leap of faith.

For example, you may have heard of Tesla’s Powerwall 2 home battery systems. These self-contained, water-cooled units have a power output of about 5 kW continuous, 7 kW peak and at press time, the product had not yet begun to ship to the U.S. In fact, Powerwall 2 is so new that agreement on its performance is hard to find. But if it works as suggested and lives up to its 10-year-warranty, this really is a game changer in terms of home storage batteries.

One Powerwall 2, operating as specified, could run a heat pump system such as mine for a few hours. Another company, Adara (see details on page 32) has a battery system that is comparable with the Powerwall 2, and ready for installation now. For the sake of analysis, however, let’s stick with the Tesla product in a hypothetical application. If you consider that one Powerwall 2 yields 14 kilowatt-hours of energy storage, and the average American household uses about 30 kilowatt-hours per day, you would probably need two of these stackable units to give you a little wiggle room.

On a sunny day, the Powerwall’s 14 kWh system is recharging even as you run the splits, and might suffice to get you through the night. But a couple of cloudy days could leave the batteries empty. The second battery buys you some time. At a cost of about $5,500 (U.S.) per Powerwall 2, plus $5000 or so for installation, that’s about a $15,000 investment on top of solar and mechanicals. Can this extra cost be rationalized?

Maybe. The answer depends on some highly complex variables that again have no definitive answers. Will the batteries last without significant degrading of power output? And can they be rebuilt affordably at the end of that time period?

Recently, Tesla dealers have been offering the batteries as part of a complete solar charging package. They can pitch the system to homeowners as a way to save a couple thousand dollars a year on utility bills, shortening payback to just a few years.

The only real “payback” is coming from the solar panels, of course. The batteries merely enable the solar in situations where net metering has gone away. Batteries depreciate in value. Solar actually becomes more valuable once the initial cost is paid off.

Hedging Solar Bets

After researching all the solar options, I arrived at what I would call a “calculated risk with a safety net scenario.” Because Maine will offer net metering for at least the near future, installing PV without a backup battery system makes the most economic sense. I’m still an early adopter. With electricity only 11 cents per kWh, and natural gas dirt cheap, I have plenty of economic disincentives suggesting I should stay the course, heat with gas and assume the world won’t change. I’ll only be saving about 2 cents per kWh (see chart) over current power prices.

But assuming that the status quo will remain stable in the face of today’s global disruptions involving fracking, infrastructure, fossil fuels and politics seems like wishful thinking, verging on ignorance.

As the henchmen of the old fossil fuel economy continue to try shoving the solar genie back into the bottle, I’ll take what I consider the lowest-risk approach. I’ll convert to solar, and set aside an account to purchase my home battery storage at the last possible moment, if solar incentives fall victim to political sleight of hand.

For those who don’t have the resources or the space to take this approach, there are other ways to get off the fossil fuel bandwagon. For example, SolarCity’s solar leasing packages allow owners to get into solar at very low initial cost, and essentially lock in electrical rates for 20 years. Getting to net zero has never been more affordable over the long run, nor more important to the future of our planetary ecosystem. 

solar and mini-split

Smart Controls for Splits

One thing I really dislike about my older natural gas heaters is lack of remote controls. When traveling or away, I have no way to check, reset or monitor conditions in my building. So it’s essential that mini splits offer flexible controls. For my mini splits, I’ll have control from anywhere via a new app from Mitsubishi Electric called Kumo Cloud. Wi-Fi-enabled apps like these are becoming much more common, although in my view the dongles you need to attach to the units are still overpriced. The Wi-Fi adapter that attaches to each of my Mitsubishi M-series mini splits retails for about $200. That’s $600 with three units.

The advantage is that I can now control the three splits as three separate zones using my smartphone and the free Kumo app. The app is available for iOS, Android or Fire OS-enabled devices, and the devices are controlled via your home wireless network with a web-based browser. The only drawback is that if your Wi-Fi router goes down while you’re away, you have no way to reset it remotely, and you’ll lose connection to your system.

One feature I’d like to see added to the app is the ability to have an alert sent to the user when temperatures in any mini split reach a certain minimum, or when connection with the units is lost or resumed (such as during a power outage). This could be useful for vacation travel to avoid frozen pipes and other problems.


Here's another good article on mini-split heating/cooling in Northern Climates by Peter Talmage.

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